Small scale HVAC systems are typically controlled by “discrete” or “fixed task” controllers. Examples of small scale HVAC systems include cabinet controllers, residential air conditioning systems, and single room controller systems such as computer rooms, archive vaults, clean rooms and laboratories. The discrete controllers used in these systems are characterized as having all control circuits disposed on a single integrated circuit board, and are typically mass produced and of relatively low cost. The discrete controllers can also be of compact design, which finds favor in certain applications such as cabinet controllers. Discrete controllers have also become increasingly sophisticated, with some units designed to accommodate not only temperature inputs, for example, but also humidity control, digital I/O for damper control in zoned heating applications, and/or supply/exhaust air flows.
The various discrete devices described above tend to differ enough from each other so as to require custom build for production. That is, a cabinet controller isn't particularly well suited as laboratory room controller because of the lack of sophistication. Likewise, the laboratory room controller is ill suited as a cabinet controller for lack of compactness and the presence of features which are not utilized, which drives up the cost. From the stand point HVAC equipment supply, it would be desirable to manufacture a system that does not require a custom build for each, i.e., one from which a wide range of devices can be produced from a common platform.
“Modular” controllers can offer the flexibility of configuring controllers for a variety of applications from a single platform. Such modularity finds advantages in large scale operations, such as office buildings and warehouses. However, for the residential market, modular controllers can be more expensive than mass produced discrete controllers. Also, modular control systems are typically less compact than discrete controller systems, which can be a detriment in certain applications.
Another aspect of discrete controllers, at least as used in the residential setting, is the implementation of high gain (“on/off”) control. Standard air conditioning units are targeted so that the duty cycle operates about 10 times an hour (i.e., within a 6-minute cycle). That is, for a 25% duty cycle, it is desired that the compressor be on for 12 minutes, off for 4½ minutes; for a 50% duty cycle on for 3 minutes, off for 3 minutes; and so on. The energy required to bring a compressor on line is about the same amount of energy required to run the compressor at steady state for about 5 minutes. Hence, if an air conditioner is cycled the desired 10 times per hour, the amount of energy required for startup of the unit is the same as for operating the unit at steady state for about 50 minutes. Put another way, if an air conditioner is operating at 50% load (i.e., 10 cycles of 3 minutes each in an hour), the amount of energy consumed is the equivalent of 80 minutes of steady state operation.
A controller that offers the compactness of discrete controllers but with the flexibility to enable production of several controller types from a common platform would be welcome. Also, a controller that mitigates the energy-consuming effect of multiple starts and stops in residential HVAC systems would also be a welcome development.